documentclass{article}
usepackage{amsmath}
begin{document}
begin{align}
  a + b &= quad text{short reason} \
  c + d &= quad text{a really long and} \
        &phantom{=} quad text{complicated reason} notag \
  e + f text{.}
end{align}
end{document}

documentclass{article}
usepackage{amsmath}
begin{document}
begin{align}
  a + b &= quad text{short reason} \
  c + d &= quad begin{aligned}[t]&text{a really long and} \
                                   &text{complicated reason}end{aligned}\
  e + f text{.}
end{align}
end{document}

documentclass{article}
usepackage{amsmath}
begin{document}
begin{align}
  a + b &= quad text{short reason} \
  c + d &= quad begin{tabular}{@{}p{0.8textwidth}}
                    really long and\
                    complicated reason
                 end{tabular} \
  e + f &=text{.}
end{align}
end{document}

documentclass{article}
usepackage{amsmath}

begin{document}
begin{align}
  a + b &= quad text{short reason} \
  c + d &= quad parbox[t]{0.35textwidth}{
                    really long and complicated\
                    reason in two lines} \
  e + f &=text{.}
end{align}
end{document}